Effects of Very High Free-Stream Turbulence on the Jet-Mainstream Interaction in a Film Cooling Flow

Abstract

Dispersion of coolant jets in a film cooling flow field is the result of a highly complex interaction between the film cooling jets and the mainstream. Understanding this complex interaction, particularly near the injection location, is critical for improving the predictive capabilities of existing film cooling models, especially when very high free-stream turbulence levels exist. This study uses a high frequency response temperature sensor to investigate the mean and fluctuating thermal field of a film cooling flow for two vastly different free-stream turbulence levels (0.5% and 20%). The high frequency response temperature sensor provides new information about the film cooling flow in terms of actual rms levels (Θ′), probability density functions (pdf’s), and frequency spectra of the thermal field. Results are presented for both free-stream conditions using round holes inclined at 35°, at a momentum flux ratio of I = 0.156 and density ratio of DR = 1.05. The mean thermal field results show severe degradation of the film cooling jet occurs with very high free-stream turbulence levels. Temperature rms results indicate levels as high as Θ′ = 0.25 exist at the jet-mainstream interface. More information is provided by the temperature pdf’s which are able to identify differences in the jet-mainstream interaction for the two free-stream conditions. With small free-stream turbulence, strong intermittent flow structures generated at the jet-mainstream interface disperse the jet by moving hot mainstream fluid into the coolant core, and ejecting coolant fluid into the mainstream. When the free-stream has large scales and very high turbulence levels, the jet-mainstream interface is obliterated by large scale turbulent structures originating from the free-stream which completely penetrate the coolant jet causing very rapid dispersion of the film cooling jet.